The Commission of the European Economic Community (EEC) has called for the provision of user access to basic Integrated Services Digital Network (ISDN) services at the `U` (2-wire) interface point within their proposals for the ONP programme. 2-wire user access to basic ISDN is seen as an inevitable step towards the cost-effective provision of the service and has been included in various presentations for over two years.
The regularly aired objection to 2-wire user access is that the maintenance facilities (loopbacks) that could be provided in the Network Terminator 1 (NT1) would not be under the control and responsibility of the Network Operator, and that therefore the determination of responsibility for rectifying a service failure would become more difficult. Objectors will quote network operators' comments to that effect from the USA, where 2-wire access is a legal requirement, and from Japan.
While objection has been vehement from some quarters, the cost and flexibility advantages of 2-wire user access to basic ISDN have been recognised and suppliers would like to offer it if only the maintenance responsibility problem could be solved.
A solution to this problem is proposed by means of an enhancement to the termination unit fitted at the subscriber's premises for Plain Ordinary Telephone Service (POTS) which will provide a simple maintenance facility and which, once fitted, will be usable both for 2-wire basic ISDN service and for POTS.
One solution for testing subscriber's lines was developed by British Telecom (BT) for 2-wire and 4-wire private circuits, where determining the responsibility for service failure is a much more urgent problem (financially) than an ordinary subscriber's access failure. This was the Automatic Loopback Equipment (ALE) the 2-wire circuit version being implemented as the Test Unit 21B. This unit, which is powered either from the line or from a local supply, responds to a command tone from the test facility by disconnecting the subscriber's equipment with a relay and transmitting a tone of 800 Hz back to the test facility for a period of 60 seconds, from which the loss of the line can be determined. The unit then releases the relay and resumes its normal transparent mode of operation.
This approach is far too complex and expensive for general deployment on customer's access lines, where a simple low cost termination unit is needed, comparable in size with the Network Termination Equipment 5 (NTE5), which can be used without modification for either 2-wire ISDN or POTS.
Another solution is described in U.S. Pat. No. 3,773,986 (Tremblay), which like the BT/ALE described above requires a local power source and wherein the test unit resides in its test mode for a predetermined period before automatically restoring the line to normal operation.
Yet another approach is described in U.S. Pat. No. 4,041,255 (Cambridge) which addresses the power problem of Tremblay by using a magnetically-latching relay and the predetermined timing constraint problem by triggering the relay `on` and `off` by current derived from high voltage signals of opposite polarity applied to the line from the central office.
A further problem which is common to the arrangements described in both Tremblay and Cambridge is that the DC control signals sent from the central office to operate the test mode have to be substantially higher than the voltages applied to the line during normal operation to avoid the possibility of spurious operation of the test mode. While this may be acceptable on lines used for normal telephone services (POTS), DC line feed voltages for ISDN services in some networks are already limited by safety regulations; so applying significantly higher voltages to the line is not a practical proposition. Moreover, since the trigger voltage (substantially higher than the normal line voltage) used in the arrangement described by Cambridge is obtained by charging a capacitor from the line, the voltage applied by the central office will be yet higher, this additional voltage margin being a factor in determining the operate/restore delay times of the test mode.
The principal problems of the prior art can be summarised as follows:
1. Fixed, automatically reset test period (BT/ALE and Tremblay): If this time is long enough for on-demand fault diagnosis, it will be far too long to permit efficient routine testing of good lines for preventitive maintenance. PA1 2. Need for local powering (BT/ALE and Tremblay): Either local powering or trickle-charging a battery is expensive and unreliable. PA1 3. Magnetically-latching relay (Cambridge): This is an expensive and sensitive device. PA1 4. Excessively high voltages needed for test mode (Tremblay and Cambridge): These high voltages cannot be provided on ISDN lines where line feed voltages for normal operation are already close to the limits set by safety regulations in many networks. PA1 a) transparent in normal use for both POTS and for 2-wire user access to basic ISDN. PA1 b) disconnects terminal apparatus from line and loops the line with a known resistance in test mode. PA1 c) simple test mode control signal from exchange. PA1 d) minimal power consumption (in normal transparent mode). PA1 e) simple, small (preferably housed in current NTE5 case ), and very low cost.